Hi! I wanted to be sure of my reasoning for this question. I thought B would be the weakest base, since it is most stabilized due to the presence of resonance, induction, and the negative charge being on N. However, structure B has resonance, in addition to the negative being on the more electronegative oxygen. 

Hi Sorry for late reply
Just saw this post.
Order is D B A C
First Go with Atom
O is more EN than N so it is less basic than N .It means D is least basic as O is holding its LP tightly.
Then B because it has so many EN atoms (3 Cl)  on it.
C is most basic because O is EDG here and making ring electron rich so deloaclization of LPs of N atom rons is less.